I must say this sounds to me like a case of good observation followed by
jumping to conclusions, something a scientist has to scupulously avoid.
Eating is the last thing a slug would be concerned with after being plunged
into a bath of hydrochloric acid and digestive enzymes. Epithelial cells
cannot be indentified as human simply by observing them under a microscope.
Any epithelial cells found in the gut of the slug could have come from
something it ate before being swallowed - or more likely were its own
epithelial cells, sloughed off in response to ingesting human digestive
juices.

Paul M.